testing/wf-config: fix build with gcc 14
Backport a fix for a rebuild error with gcc 14.
0.9.0 is available, but it is marked as for wayfire 0.9.x. It was unclear if it were backwards compatible to 0.8.0. Since the wayfire aport is still at 0.8.0 and 0.9.0 compatibility with wlroots 0.18.x is in progress at this time, the MR backports the gcc 14 patch included in 0.9.0.